In recent years, liquid crystal display devices have rapidly become popular in replacement of cathode-ray tube (CRT) based display devices. Such liquid crystal display devices have been in widespread use in devices such as liquid crystal televisions, monitors, or portable phones, each of which takes advantage of features, of the liquid crystal display devices, such as low power consumption, small thickness, and light weight. One way to further take advantage of such features is to improve an illuminating device (so-called backlight) disposed behind a liquid crystal display device.
An illuminating device is largely classified into a side light type (also referred to as an edge light type) and a direct type. A side light type illuminating device is arranged such that a light guide plate is provided behind a liquid crystal display panel and that a light source is provided at a lateral edge of the light guide plate. Light emitted from the light source is reflected by the light guide plate so that the liquid crystal display panel is indirectly and uniformly irradiated by the light. With this arrangement, it is possible to realize an illuminating device which has a reduced thickness and high uniformity of brightness although the brightness is low. Accordingly, a side light type illuminating device has been mainly applied to medium-sized and small-sized liquid crystal display devices such as a portable phone and a laptop personal computer.
Examples of such a side light type illuminating device include a surface light-emitting device disclosed in Patent Literature 1. According to the surface light-emitting device, a reflection surface of a light guide plate is provided with a plurality of dots so that light can be uniformly emitted from a light-emitting surface. The surface light-emitting device is arranged such that a density of dots in corner parts of the reflection surface is made higher than the other parts. This is because light is not propagated to the corner parts due to a directional characteristic of each of light sources and the corner parts therefore become darkened.
A direct type illuminating device includes a plurality of light sources disposed behind a liquid crystal display panel so that the liquid crystal display panel is directly irradiated by light. With this arrangement, it is possible to easily achieve high brightness even in a case of a large-sized screen. Accordingly, such a direct type illuminating device has been mainly applied to a large-sized liquid crystal display device having a screen size of 20 inches or more. Unfortunately however, a currently available direct type illuminating device has a thickness of approximately 20 mm to 40 mm. This poses an impediment in realizing a further reduction in thickness of a display device.
A further reduction in thickness of a large-sized liquid crystal display device can be achieved by reducing a distance between the respective plurality of light sources and a liquid crystal display panel. Note however that it is impossible to obtain brightness uniformity of an illuminating device unless the number of light sources increases. However, an increase in the number of light sources causes an increase in cost. In view of this, there is a demand for development of an illuminating device which achieves a reduction in thickness and high uniformity of brightness without increasing the number of light sources.
Conventionally, in order to address the problems, attempts, in which a reduction in thickness of a large-sized liquid crystal display device is achieved, have been made by disposing a plurality of side light type illuminating devices.
For example, Patent Literature 2 discloses a surface light source device in which tandem-arranged plate-like light guide blocks are provided so as to partially overlap each other, and primary light sources each supplying primary light to a corresponding one of the plate-like light guide blocks. With this arrangement, it is possible to secure a large light-emitting area with a compact structure. Accordingly, such a surface light source device can be suitably applied to a large-sized liquid crystal display device.
Further, Patent Literature 3 discloses an illuminating device in which a plurality of polygonal light-emitting modules are aligned. Each of the plurality of polygonal light-emitting modules includes a light-emitting body, and a light source such as an LED that is disposed on an end part of the light-emitting body.
Such an illuminating device in which a plurality of light-emitting units, each of which is constituted by a combination of a light source and a light guide plate, are disposed is called a tandem type-illuminating device.